The 13C-NMR and catalytic properties of selectively guanidinated cytochromes c will be measured to deduce the surface of cytochrome c involved in complexation and electron transfer with cytochrome c peroxidase in solution. These studies will be complemented by X-ray analysis of crystals of cytochrome c peroxidase and its complex with cytochrome c. Proposed mechanisms of electron transfer will be tested by specific chemical alteration of participating residues.